Power transmissions networks can be made of AC systems, DC systems, or a combination of the two. AC power networks have conventionally been used throughout the world. However, DC power networks have certain advantages. DC power networks are easier to design and implement because they introduce no reactance into the power system. Higher efficiencies from generators can be achieved in DC systems because only real power is transmitted. Additionally, parallelization of power supplies is simple because no synchronization is required when additional supplies or loads are brought onto the network.
Therefore, in power networks that experience large swings in load on the generators and require reliable operation, a combination of DC systems and AC systems is beneficial. One example of such a power network is found on drilling platforms or vessels to operate onboard thrusters. Drilling vessels are not anchored in the ocean but are dynamically controlled to maintain a desired position in the ocean. Thrusters are propeller drives that can have variable rotation speed and azimuthal angle of the blades. They are used to maintain a position within specified tolerances of a drilling apparatus. These thrusters are operated by a power supply onboard the drilling vessel. Any failure of the power supply can lead to displacement of the vessel out of the tolerances of the drilling apparatus. In such a case, the drilling apparatus would need to be mechanically decoupled and recoupled after the power supply is restored and the position of the drilling vessel is corrected.
One method of facilitating a reliable power supply is to utilize a DC bus for powering thrusters and other components. Such a power transmission system is demonstrated in FIG. 1. In such a system, the power supply is generally made of AC generators coupled to an AC-to-DC converter, such as AC-to-DC converter 112. The AC-to-DC converter places power from the AC generators on an intermediate DC bus. Each motor or thruster, as well as other devices utilizing the intermediate DC bus, on board the drilling vessel is coupled to the intermediate DC bus through a DC-to-AC converter.
FIG. 1 is a block diagram illustrating a conventional DC voltage bus coupling multiple AC voltage generation systems to various loads. Power system 100 includes generators 102. The generators 102 are coupled to an AC bus 104 through isolators 106. The isolators 106 allow the generators 102 to be removed from the AC bus 104 when they are not used or are malfunctioning. The AC bus 104 is coupled to a transformer 108 to condition power for transmission to a line 110. An AC-to-DC converter 112 is coupled to the line 110 and converts AC power on the line 110 to DC power for output onto an intermediate DC bus 120. Coupled to the DC bus 120 are DC-to-AC converters 130. The DC-to-AC converters 130 convert DC power on the DC bus 120 to AC power that most devices are designed to use. Coupled to the DC-to-AC converters 130 is a line 132 to which loads may be connected. A power dissipating device 134 is coupled to the line 132, and the power dissipating device 134 may be, for example, a thruster. Additionally, a transformer 135 is coupled to the line 132 to condition power for a load 136. The load 136 may be, for example, a light bulb.
Another example of the motor 134 may be the draw works onboard a drilling platform. The draw works is a machine that reels out and reels in the drilling line and conventionally includes a large-diameter steel spool, brakes, and a power source. Operation of the draw works to reel in drilling line may require the full capacity of the ship-board generators. However, there are operations conditions where the draw works may consume zero power. In reverse operation, the draw works may generate power that is placed back on the line 132 while gravity assists reeling out of the drilling line. The power load changes may occur nearly instantaneously.
Rapid changes in the load on the generator require the generator to increase power output to generate the power demanded by the load. Diesel generators are designed to consume fuel at an optimized rate in a small range of the available power output. Diesel fuel costs are the highest expense incurred by operating a diesel generator over its lifetime. Therefore, an operator desires to keep the generator operating in the power output range optimized for fuel consumption.
Turning now to FIG. 2, a power output curve for a diesel generator are examined. FIG. 2 is graph illustrating the operation of a diesel generator. A curve 220 represents fuel consumption in kilograms per kilowatt-hour of the diesel generator at various engine loads (power output). A range between 0 and 100 percent of rated output demonstrates a variation in the kg/(kw/hour) ratio, or efficiency of fuel consumption In order to operate efficiently a range 230 of power load on the diesel generator should be maintained. If the load increases or decreases, the engine fuel consumption and efficiency changes.
In addition to fuel consumption issues, scrubbers on diesel generators that reduce the dangerous exhaust are sensitive to the volume of exhaust. Rapidly varying engine power changes the rate of flow of exhaust and chemical components of the exhaust. Because the scrubber is designed to operate optimally on a continuous and stable flow of exhaust, emissions output may not be minimized if the power load varies rapidly.
Further, dynamic performance of diesel generators is limited. That is, diesel generators may not increase power output rapidly enough to match an increasing power load on the diesel generator. Conventionally, additional diesel generators would be brought online if the rate of increase of power load exceeds the rate of increase of diesel generator power output. Neither diesel generator is operating efficiently and results in increased fuel consumption and express capacity when the power load peaks.
Referring now to FIG. 3, generators and power loads will be examined in a conventional power plant. FIG. 3 is a block diagram illustrating power distribution on a conventional power plant 300. The power plant 300 includes an AC generator 302 coupled to a switchboard 308 through an AC line 306. The switchboard 308 is coupled to multiple loads. For example, typical shipboard and drilling loads are represented by a power dissipating device 312 coupled to the switchboard 308 by an AC line 310. Additionally, the switchboard 308 is coupled to an AC-to-DC converter 318. The AC-to-DC converter 318 is coupled to an AC line 316 and a DC line 320. Additional loads may be coupled to the DC line 320. For example, a light 322 may be coupled to the DC line 320 or a DC-to-AC converter 324. The DC-to-AC converter 324 couples to additional AC loads such as a power dissipating device 326. The power dissipating device 326 may be a draw works as described above or a motor. Each of the loads 312, 322, 326 produces different power loads on the AC generator 302. The effect on the AC generator 302 will now be examined.
FIGS. 4A to 4E are graphs illustrating power consumption in a conventional power plant such as FIG. 3. A line 402 in FIG. 4A indicates power consumption at the power dissipating device 312. Shipboard loads such as the power dissipating device 312 operate as a constant load over long periods of time such as hours on the AC generator 302. The line 402 is positive indicating consumption of power. A line 404 in FIG. 4B indicates power consumption at the power dissipating device 326. Draw works such as the power dissipating device 326 operate as a varying load, which may change rapidly such as in milliseconds, on the AC generator 302. The line 404 varies between positive and negative values indicating the load consumes power at some times and produces power at other times. A line 406 in FIG. 4C indicates power consumption at the light 322. The light 322 operates as a constant load over long periods of time such as hours on the AC generator 302.
Total power transferred through the AC-to-DC converter 318 is represented by adding the line 404 to the line 406 and is shown in a line 408 in FIG. 4D. The line 408 is total power consumption with respect to time of the DC line 320. Total power delivered by the AC generator 302 is shown in a line 410 in FIG. 4E and is a sum of lines 408, 402. In the conventional power plant 300 power delivered by the AC generator 302 varies in time. This leads to undesirable qualities exhibited by the AC generator 302 as indicated above including inefficient fuel consumption and poor exhaust scrubbing.
Thus, there is a need for a power plant design that produces a substantially constant load on the AC generators and increases dynamic performance.